Access to various subterranean formations containing mineral deposits may be achieved by means of one or more wells drilled from the surface into the deposit. Where the mineral deposits are in liquid form, the wells are typically produced by pumping the liquids to the surface. Many of the hydrocarbon reserves remaining in the world consist of heavy oil and bitumen which reside in oil sands deposits. Recovery from the formations containing these deposits is often uneconomical due to the poor well productivity achieved using current technology.
Conventional drilling technology provides for the drilling of wellbores from the surface to a predetermined depth beneath the surface into the subterranean formation. While most wellbores have traditionally been drilled substantially vertically or perpendicular to the surface, current drilling technology also provides for the drilling of slant wellbores at an angle to the surface. Recent advances in drilling technology allow for the drilling of a wellbore beneath the surface with a portion of the wellbore having a longitudinal axis either parallel to the surface or at a slight angle to it. These wellbores, often referred to as horizontal wells, allow placement of the near horizontal segment of the wellbore within the formation. Horizontal wells are typically formed by drilling a vertical wellbore downward to the desired depth beneath the surface, turning the wellbore toward the horizontal and then extending the wellbore horizontally into the formation. In such a circumstance, the entire horizontal segment of the wellbore in contact with the hydrocarbons performs the function of collecting the hydrocarbons from the formation for production to the surface.
Horizontal wellbores are therefore advantageous as compared to conventional vertical wellbores as horizontal wellbores generally allow greater contact between the wellbore and the hydrocarbon bearing formation. Generally, the longer the horizontal wellbore, the greater the contact with the formation and the greater the portion of the wellbore collecting hydrocarbons from the formation. Thus, horizontal wellbores allow for improved drainage and improved productivity from the formation.
However, several difficulties have been encountered with the use of horizontal wellbores in recovering heavy oils and other viscous liquids. One difficulty is that the length of the horizontal wellbore contributing fluids may be limited by the production system capacity. Increasing the length of the wellbore beyond the optimum for a specific production system may result in a decreased production of liquids per unit length of the wellbore.
In addition, in most cases where horizontal wellbores are produced by artificial lift means, conventional pumping systems such as progressing cavity pumps are typically positioned with the pump intake above the horizontal wellbore and the liquid level is typically several meters above the intake. This places a hydrostatic head on the liquids within the horizontal wellbore which tends to impair the inflow potential of the horizontal wellbore. The inflow potential is directly related to the magnitude of the pressure differential existing between the wellbore and the surrounding formation.
Finally, conventional production systems using downhole pumps create a point source drawdown of the wellbore. As a result, productivity with these systems is limited by the inherent flow-induced pressure losses which occur along the wellbore, particularly in the case of viscous liquids. This promotes the development of a non-uniform pressure profile along the length of the wellbore which may result in non-uniform inflow along the wellbore and in either premature water breakthrough or significant sand influx near the start of the wellbore. These problems, in turn, can lead to the shut-in of the well.
Several concepts have been developed to improve productivity through the use of u-shaped wells which include a substantially horizontal wellbore segment located in the formation for collecting liquids. For example, Canadian Patent No. 481,151 issued Feb. 12, 1952 to L. Ranney discloses the drilling of a downwardly inclined hole from the surface towards a coal seam or other mineral deposit. As the hole approaches the deposit, it is deflected upwards to become substantially horizontal and parallel with the deposit. After continuing horizontally for an indefinite distance, the hole deflects upwards again and emerges at the surface. Air, oxygen or other fluids are supplied to the horizontal portion of the hole through the downwardly inclined portion, as necessary, and the mineral liquids are removed through the upwardly inclined portion. The liquids are removed through the upwardly inclined portion by pumping the liquids out in the normal or conventional fashion.
U.S. Pat. No. 4,037,658 issued Jul. 26, 1977 to D. J. Anderson also discloses the drilling of an injection shaft and a recovery shaft which both extend from the surface to a tar sand formation. A hole is formed through the tar sand formation between the shafts and a tubular member is inserted therein. To recover petroleum from the formation a hot fluid is flowed through the tubular member which heats the viscous petroleum surrounding the member and forms a potential passage for fluid flow through the formation. A drive fluid, such as steam, gas or water, is then injected from the injection shaft into the formation through the passage to promote the flow of petroleum toward the recovery shaft. However, again, the petroleum is recovered from the recovery shaft using means for lifting the petroleum from the interior of the recovery shaft, such as a pump.
U.S. Pat. No. 4,532,986 issued Aug. 6, 1985 to D. S. Mims et. al. discloses two intersected wells. A horizontal well is drilled to lie generally horizontally and adjacent to the lower border of a hydrocarbon containing layer. The horizontal well is perforated along its length and has a production end communicating with the surface and an injection end, the ends being separated by a barrier. A vertical well intercepts the injection end of the horizontal well. To recover the hydrocarbons within the formation, a stream of hot stimulating fluid, such as steam, is carried to the injection end of the horizontal well via the vertical well. Once in the horizontal well, the fluid is injected into the formation through the injection end, where it liquifies the hydrocarbons. The liquified bitumen then moves into the production end of the horizontal well where it is removed. The means for removal from the production end are not described.
U.S. Pat. No. 3,986,557 issued Oct. 19, 1976 to J. H. Striegler et. al. and U.S. Pat. No. 4,445,574 issued May 1, 1984 to R. R. Vann also utilize a u-shaped system of wellbores. However, these patents do not use conventional pumping systems to recover the hydrocarbons collected in the wells.
U.S. Pat. No. 3,986,557 discloses the drilling of a continuous wellbore having a second section, contained within a subterranean tar sand formation containing viscous bitumen, and a first and third section extending the second section to the surface. A heated fluid is circulated through the wellbore via the first section and the mobilized bitumen is recovered via the third section. The patent describes the means of recovery of the mobilized bitumen as being the driving force of the circulating heated fluid. No other means are described.
U.S. Pat. No. 4,445,574 similarly discloses the drilling of a continuous borehole extending from an inlet on the surface to an outlet on the surface having a horizontal portion extending through a pay zone containing hydrocarbons. Production is achieved by flowing a fluid into the inlet of the borehole to flow through the entire borehole thereby forcing the production of hydrocarbons collected in the horizontal portion to the surface.
None of the systems described above appear to have been used in practice by industry because of being either physically impractical or uneconomical.
Therefore, there remains a need in the industry for a relatively uncomplicated method and apparatus for producing liquids from a subterranean formation using a well having a collecting wellbore for collecting the liquids located at least partially within the formation, which improve both the inflow potential and the uniformity of the inflow along substantially the entire length of the collecting wellbore.